SEOUL (REUTERS) - North Korea attempted and failed to launch what experts believe was an intermediate range ballistic missile on Friday (April 15) in defiance of UN sanctions and in an embarrassing setback for leader Kim Jong Un.

The failed launch, as the reclusive country celebrates the "Day of the Sun" on the birthday of Mr Kim's grandfather Kim Il Sung, follows the North's fourth nuclear test in January and a long-range rocket launch in February, which led to new UN sanctions.

But the North has nevertheless pushed ahead with its missile programme, supervised by Mr Kim.

The US-based 38 North website, which specialises in North Korea, has said there has been activity at the country's nuclear site based on satellite imagery, and on Wednesday it said the possibility of a fifth nuclear test "could not be ruled out".

Friday is the birthday of North Korea's late founding president Kim Il Sung, which is widely celebrated. In 2012, it was marked by a long-range rocket launch attempt which also failed.

The US Defence Department said in a statement that the launch at 5.33 am Korea time (4.33 am Singapore time) was detected and tracked by the US Strategic Command which also assessed it had failed.

"We call again on North Korea to refrain from actions and rhetoric that further raise tensions in the region and focus instead on taking concrete steps toward fulfilling its international commitments and obligations," a US State Department official said.

It was likely a Musudan, South Korea's Yonhap news agency said, an intermediate-range ballistic missile with a design range of more than 3,000 km that can be fired from a road mobile launcher but which has never been flight-tested.

A South Korean defence ministry official said Friday's launch was believed to have been a failure but declined to identify the type of the missile launched.

The United States, which has 28,000 troops stationed in South Korea, said on Thursday that it was aware of reports that North Korea was preparing to test intermediate-range missiles and was closely monitoring the Korean peninsula.

"Timing wise, today's missile was a cannon salute on the Day of the Sun, leading up to the party congress, but now that it has failed, it is an embarrassment," said Mr Chang Gwang Il, a retired South Korean army general.

The North is scheduled to hold its ruling party congress in early May, the first such meeting in 36 years.

The North could not completely ignore the sanctions, backed by its lone major ally China, but considered it the right time to attempt a missile launch to send a message to the world "we don't surrender to sanctions", Mr Chang said.

Some experts had said North Korea may choose to test-fire the Musudan as it tries to build an intercontinental ballistic missile designed to put the mainland United States within range.

North Korea, which regularly threatens to destroy South Korea and the United States, often fires missiles during periods of tension in the region or when it comes under pressure to curb its defiance and abandon its weapons programmes.

The reclusive North and rich, democratic South are technically still at war since their 1950-53 conflict ended in a truce, not a peace treaty.